1. Field of the Invention
This invention relates generally to current sensing circuits in which current flowing through a conductor is measured, and more specifically, those current sensing circuits which employ a small sensing resistor in series with the conductor.
2. Description of the Related Art
Current sensing circuits commonly found in the art employ a sensing resistor, generally a very small value resistor, in series with a conductor, and current flow is determined by observing the voltage drop across the sensing resistor. The sensing resistor is generally kept as small as possible to avoid wasted power, yet large enough to provide an adequate signal voltage level. However, replicating this signal at ground potential can be difficult to arrange.
A common practice has been the employment of a balanced bridge voltage divider to accomplish translation from the voltage across the sensing resistor to an output voltage which is proportional to the current flowing through the sensing resistor. However, as the supply voltage becomes greater, the fraction of the voltage across the sensing resistor available at ground reference becomes smaller. If the resistors in the balance bridge start at precisely the proper values and track each other every closely over both time and temperature, amplification may then be used to recover the signal voltage.
However, any imbalance in the bridge shows up as an offset voltage which may introduce errors in the output signal. This, of course, is a very serious problem if the offset voltage is an appreciable fraction of the desired signal. Because the offset voltage is of unknown sign, it may make the output signal voltage appear either larger or smaller than the intended value.